1. Field of the Invention
A common requirement of industry and commerce is to convert a material manufactured in continuous web form into an endless belt. One of the most common methods for accomplishing this purpose is called a butt joint. To make such a joint, a suitable length of the web material, hereinafter called the prime web, which is desired for the bulk of the belt, is cut and looped so that the cut ends adjoin, and a narrow space between the cut ends is filled with a suitable adhesive. If, as is normal, the web is relatively thin compared with its width and length, a reinforcing material is usually added to strengthen the butt joint so formed.
The reinforcing material considered here and called a tape or patch material is itself a relatively thin web from which is cut a strip of sufficient width to extend for an appropriate distance on each side of the butt joint to be reinforced and of sufficient -ength to extend across the width of the beit to be formed, at whatever angle is desired. The patch material is placed on one side of the endless belt formed by a simple butt joint and adhesive-y attached to the prime web on at least one side thereof in such a position that part of an integral piece of the patch material is attached to both of the ends of the length of primary web which abut to form the butt joint. The adhesive which attaches the patch to the prime web is normally continuous with the adhesive which fills the space between the ends of the prime web.
A patch reinforced butt joint of the type described here is illustrated in FIG. 1 of U.S. Pat. No. 4,215,516 to Huschle et al. (hereinafter cited simply as Huschle). The prime web as described herein is designated 10 in the Huschle Figure, with ends 12 and 13 abutting at joining line 14. Item 16 of the Huschle Figure, regarded here simply as a web without any restriction as to its detailed structure, is the patch material as defined here.
The instant invention is particularly adapted to provision of patch materials suitable for use in forming endless belts of coated abrasive products. Because coated abrasive belts are commonly used at fairly high tensions to assure mechanical control, patch materials for such belts must be resistant to tensile breakage. Because many machines which use coated abrasive belts have only limited capacity to adapt to any stretch or elongation of the belts which may develop during use, patch materials for coated abrasives should resist stretch under the tensions applied. Because many coated abrasive belts are used to achieve precise dimensionai control of the objects worked by them and because achievement of such control is often promoted by minimal variation of the thickness of a coated abrasive belt along its length, it is advantageous tor patch materials to be thin, so that the part of the belt to which the patch is attached will not be adversely thicker than the remainder of the belt. As shown in the already referenced Huschle Figure, part of the thickness of many coated abrasive prime webs can be removed in the zone of the joint to accommodate the patch. However, it will be appreciated that such removal of material can not be extended beyond a certain limit without seriously weakening the prime web. Even when precise dimensional control of workpieces is not required, substantial variations in thickness or stiffness of a coated abrasive belt may cause unacceptable surface blemishes, known in the art as "chatter marks", on the workpieces finished with sucn belts. It is an object of this invention to provide patch materials capable ot meeting all these diverse requirements, in a manner superior to those patch materials previously known, at acceptable cost.
2. Description ot the Prior Art
A wide variety of patch materials have been employed in the coated abrasive industry, but the most common ones in recent years have been woven fabrics and biaxially stretched polyethylene terephthalate films. The tabrics have varied widely in construction, but have usually featured synthetic multifilament yarns as the primary strengtn members in either the warp or fill direction of the fabric. Normally such fabrics are significantly weaker in whichever direction is perpendicular to tne one with most strength. Tnis promotes efficient use of textile yarns for the purpose, because in a joined belt only those yarns of the patch material which lie in or near the machine direction of the belt will normally make any significant contribution to the strength and stretch resistance of the joint.
One of the persistent difficulties with the use of woven cloth as a patch material is that most coated abrasive belts have the joints formed at an angle distinctly different from the perpendicular to the edge of the be-t. The angle, measured trom the edge on the "longer" side of the coated abrasive prime web when considering only one end of the web, is most often between 35 and 60 degrees, although it can range from near zero to the maximum possible ninety degrees. Conventional weaving produces fabric in which all yarns are either parallel or perpendicular to the direction of weaving. If such a fabric is to be used most efficiently as a patch material for coated abrasive belts joined at an angle significantly ditterent from perpendicular, the patch fabric must be cut in nonrectangular parallelogram shape for each patch, so that an approximately equal width ot patch material will lie on each side ot the joint across the whole width of the belt and there will be no overhang of patch material at the belt edges. Also, for maximum efficiency in the use of the often expensive textile yarns which provide the strength of the patch material fabric in its strongest direction, the patch must be oriented when cut so that these yarns will lie in or near the machine direction of the belt to be joined when applied to the belt.
On a commercial scale, which currently requires the manufacture of hundreds of millions of coated abrasive belts in the United States every year, the proper shaping of the patch material is accomplished by slitting it to the width appropriate for use, usually 10-25 mm, from fabric coated with adhesive in much wider width such as one to two meters. As will be apparent from the paragraph above, such slitting must be done at a bias angle to the original machine direction of the fabric to be made into patch in order to achieve the proper orientation of yarns when the final slit patch material is applied to the belt to be joined. Various ingenious means, including biased slitting of fabric joined into a wide endless belt or even woven originally and coated in tubular form, have been described in the prior art to accomplish efficient preparation of patch material with the correct final orientation of its major reinforcing yarns, but the process remains considerably more complex and expensive than desirable.
Another disadvantage ot woven fabrics as the primary strength members for patch material was recognized and largely overcome in the Huschle patent already cited. The problem is described at column 3 lines 29-51 of the Huschle specification. Briefly, it involves abrasion of yarns of the fabrics against each other as coated abrasive belts are flexed during use. Huschle describes a patch material in which textile yarns are used for reinforcement without weaving the yarns into a fabric first. In this way the expense of the weaving process and of the cross direction fibers required for weaving are eliminated, and better use of the strength of the yarns which are used is achieved, because these yarns are not distorted in the manner which is inherent in weaving. Instead, the yarns are laid out in parallel array and held in place by the same adhesive which will later bond the yarns to the belt to be joined. Sometimes a metal foil, plastic film, or other auxiliary web is used either temporarily or permanently to aid in handling the Huschle type patch material.
Although the Huschle patch material, called "unidirectional tape", has many advantages and has attained substantial commercial use, it still has certain undesirable characteristics. One of these is relatively low resistance to splitting. Because all the yarns in the unidirectional tape are parallel to each other within a rather small tolerance, none of the yarns can contribute much resistance to mechanical forces directed parallel or nearly parallel to the yarns, such as the forces which initiate splitting ot the patch along a line parallel to the direction of belt travel during use. Furthermore, once a split is initiated, it can readily propagate in the space between two nearest neighbor yarns of the Huschle patch, because propagation in such a direction never encounters a strong yarn which might terminate it, if breaking the yarn were necessary for the split to continue propagating. Secondly, the methods described by Huschle for commercial scale preparation of patch material still require biased slitting or some similar expedient as described above in order to achieve efficient yarn orientation in the patch when it is used. It is an object of the present invention to overcome these two disadvantages of the product and process described by Huschle while retaining most or all of their advantages.